It is desirable to encode television signals to provide secure transmission in applications such as cable TV, satellite transmission and subscription broadcast television. Known coding arrangements may be considered, generally speaking, to be of either the amplitude alteration type or of the time sequence alteration type. Amplitude encoders include, illustratively, those which alter the vertical or horizontal synchronizing pulse amplitude or which modulate the video signal with an encoding waveform (e.g., a sinewave) and have an advantage in that the decoder may be relatively simple and inexpensive. A disadvantage of amplitude encoding is that, generally speaking, the codes are rather easily recognized and deciphered, thus creating a potential "black market" for unauthorized decoders. From a technical standpoint, the amplitude distortion imparted to the video signal may not be completely removed by the decoder and critical decoder adjustments may be required to reduce the residual distortion of the decoded signal to acceptable levels. Other problem areas associated with scramblers of the amplitude alteration type include signal-to-noise ratio degradation and loss of dynamic range.
Encoders of the time sequence alteration type reorder the video signal sequence so as to create a non-standard signal sequence which may be charged at random so as to provide a very high level of security. As an example, in the system described by T. A. Banning, Jr. in U.S. Pat. No. 3,379,825 entitled SUBSCRIPTION TELEVISION SYSTEM HAVING RASTER DISTORTION which issued Apr. 23, 1968, a video signal is scrambled by reversing the horizontal scan direction of a camera and descrambled by reversing the scan direction of the kinescope of a receiver. In the camera conventional horizontal sync pulses are added to the reversed "active" video signal. Such an arrangement avoids the amplitude and sync distortion problems characteristic of amplitude alteration and sync suppression systems but requires the use of special television receivers and cameras having bidirectional horizontal deflection circuits.
It is known that the disadvantage of the Banning Jr. system (i.e., the need for a special camera and receiver) may be overcome by employing video memories to facilitate line reversal and re-reversed rather than changing the actual direction of scan in a camera or receiver. Such a system is described by Block et al. in U.S. Pat. No. 4,405,942 entitled METHOD AND SYSTEM FOR SECURE TRANSMISSION AND RECEPTION OF VIDEO INFORMATION, PARTICULARLY FOR TELEVISION which issued Sept. 20, 1983. In an embodiment of the Block et al. system a horizontal line of video signal is stored in a bidirectional shift register. When shifted in the "forward" direction, the register functions as a first-in first-out (FIFO) memory and provides a video output signal in which the sequence of picture elements is identical to the original line. The sample sequence is reversed by shifting the register in the "reverse" direction so as to operate as a first-in last-out (FILO) memory. As in the system of Banning Jr., Block et al. scramble the active video portion of the video input signal and then multiplex conventional sync and blanking signals with the scrambled active video portion to form a complete line of the processed output signal.